One of the problems encountered in this field may be found in the detection of fluorescence generated by a plasma. Indeed, a material, whether it is in solid, liquid or gaseous form may, after excitation by a laser pulse or any other excitation system, be transformed into plasma (mixture of free electrons, ions, atoms and molecules). If excitation of the material is significant enough, other well-known physical phenomena come into play such as cascade ionizations and collisions between free electrons. The effects increase the temperature of the plasma produced. The Bremsstrahlung of the moving electrons (inverse Brehmsstrahlung effect) therefore gives a white light emitted by the plasma. Analysis of the radiative deexcitation of the atoms and ions therefore allows the composition of the latter to be traced back via a spectral analysis of the light emitted by the plasma.
Detection and collection of fluorescence are conventionally carried out with an optical fiber placed at the level of the plume of the plasma. The spectrometers are therefore based on an optical design based on an echelette diffraction grating, an adjustable entrance slit, and spherical mirrors allowing the object and image points to be conjugated on a CCD camera. Reading software then allows the spectrum of light entering in the device to be displayed irrespective of the temporal or spatial coherence of the light. Thus, the incoherent light, emitted for example by Brehmsstrahlung radiation may distort the measurements and produce inaccuracies on the decisions. The method of “sorting” the emission spectrum from the incoherent white light is not satisfactory.
A number of publications disclose spectrometric methods by interferometry such as, for example, CA 2 302 994, but they have not tried to solve the problem of the incoherent white light.
An article from NASA published in the “Optics and Photonics News” journal in January 2004 describes a Fourier transform spectrometer based on a Michelson interferometer modified by replacing the mirrors with diffraction gratings. Such a spectrometer has a greater robustness to accelerations, but does not consider the problem of white light noise for detecting atomic lines.